pic
I am using the PIC24F to make a simple stop watch and it was all going well.
I am using TIMER1 and various interupts.
The problem I am having is that I cannot seem to get TIMER1 to have a period of 1ms. at the moment I am putting 6250-1 into PR1 and it isn't accurate enough.
Does anyone know the formula or method I should use to work out what to put into PRI? I think the chip is currently running at 8MHz
Any help would be fantastic!
The directive FOSC_INTOSCIO means that the part is using its internal oscillator, and the lines normally used for the crystal are set as I/O lines. That's why your crystal isn't oscillating.
This device has an internal oscillator frequency of approximately 4MHz, so it's quite possible that your ISR is executing faster than your eye can perceive the LED flashing.
You will want to use something like LP_OSC instead of FOSC_INTOSCIO (I don't have that exact header file, so I cannot tell you which exact one to use).
Further to the comment below: according to the datasheet, the choices for the timer clock source are: an external clock from RB6 or the internal oscillator / 4. What this means is that you can use the internal 4 MHz oscillator for the main clock, and bring a clock signal to RB6 for the timer. However, since you're not using the built-in oscillator hardware, you'll have to externally generate a clock from the crystal and apply it to RB6.
Luckily Microchip explains how in the datasheet:
If it only sometimes starts, it is probably a hardware problem. Check that the power supply rise time is within spec for the PIC. If not, a external reset circuit may be required. The power up timer and brown out detect (if there is one) may also help.
It doesn't seem that the firmware is all that relevant to this, but there are a number of issues with it. Hard coded constant, like 7373, magically picked out of the air are a really bad idea. Also, your 3-byte increment routine in the interrupt is broken. It's funny how you have to stand on your head to increment 3 bytes in C when this would be simpler in assembler. I haven't done a interrupt routine in C30, so I'm not sure about this, but I don't see any obvious way the compiler is supposed to know this is a interrupt routine and therefore end with a RETFIE instruction instead of RETURN. Isn't there some pragma you have to mutter to the compiler for it to produce a valid interrupt routine?
Best Answer
PR1 should be set to the the number of timer counts you want in a period, minus 1. To calculate that you need to know the instruction cycle rate. You say the chip "runs at" 8 MHz, so I guess that means the instruction cycle rate, not the clock rate. In that case there are 8000 instructions per ms, so PR1 should be set to 7999. Keep the prescaler at 1.